The present invention relates generally to glass compositions for liquid crystal display devices, and more specifically to alkaline earth boroaluminosilicate glasses exhibiting the hardness, chemical durability, and high-temperature stability required for use in the forming of glass sheet for liquid crystal display devices.
Alkaline earth boroaluminosilicate glasses constitute a very well known family of glass compositions. U.S. Pat. No. 2,393,449, for example, discloses alkaline earth boroaluminosilicate glasses which are substantially free of alkali metal oxides, and which therefore exhibit high dielectric constants and low power factors. These glasses have compositions comprising about 10-30% BaO, 5-20% Al.sub.2 O.sub.3, 22-80% B.sub.2 O.sub.3, and up to about 55% SiO.sub.2 by weight, and are particularly suitable for use as glass dielectric layers in capacitors and other electronic devices.
Other known alkali-free silicate glasses, more recently developed, are the alkaline earth aluminosilicate glasses employed as lamp envelopes in tungsten-halogen lamps. Tungsten-halogen lamps require glasses having relatively high strain points, low coefficients of thermal expansion, and high viscosity at the liquidus, so that they may be formed utilizing conventional glass tube drawing equipment and will withstand the relatively high lamp operating temperatures required. Examples of this type of glass are reported in U.S. Pat. No. 3,978,362, which discloses glasses comprising about 58-63% SiO.sub.2, 13-16% Al.sub.2 O.sub.3, 14-21% CaO, 0-5% MgO, and 0-7% BaO, with the total of CaO, MgO and BaO constituting at least 19% by weight.
Additional families of aluminosilicate compositions designed specifically for use in tungsten-halogen lamps have also been reported. U.S. Pat. No. 4,302,250 discloses glass compositions comprising 64-68% SiO.sub.2, 11-14% CaO, 16.5-18.5% A1.sub.2 O.sub.3 and 2-6.5% total of SrO and BaO, wherein SrO may range from about 0-4% and BaO about 0-5% by weight of the composition. These glasses exhibit strain points in excess of 750.degree. C., and offer high viscosity at the liquidus in combination with a relatively low liquidus temperature.
U.S. Pat. No. 4,394,453 discloses glasses comprising about 60 .+-.1.5% SiO.sub.2, 17.+-.1% Al.sub.2 O.sub.3, 5.+-.0.8% B.sub.2 O.sub.3 11.4.+-.0.8% CaO, and 7.5 .+-.0.8% MgO. These glasses reportedly exhibit improved thermal stability and viscosity characteristics which are needed for glass tube manufacture by the Vello tube drawing process. U.S. Pat. No. 4,409,337 discloses glasses for tungsten-halogen lamps consisting essentially of about 56-59% SiO.sub.2, 16-17% Al.sub.2 O.sub.3, 4.5-5.25% B.sub.2 O.sub.3, 7.5-9.25% CaO, 5.5-6.25% MgO, and 5-9% BaO. A critical feature of the latter compositions is control of the BaO content to effect a lowered liquidus temperature, with ZnO optionally being present to modify thermal expansion.
A current application of particular interest for alkaline earth aluminosilicate glasses is in the manufacture of flat glass substrates for flat panel display devices. U.S. Pat. No. 4,634,683 generally describes the glass properties required for this application, and the processing used to incorporate such glasses into flat panel displays. Glasses disclosed as suitable for this use consist essentially, in mole percent, of approximately 68-80% SiO.sub.2, 18-26% Al.sub.2 O.sub.3, and 2-6% total of BaO and SrO.
Related glasses for flat panel displays are described in U.S. Pat. No. 4,634,684, wherein glasses consisting essentially, in mole percent, of about 77-82% SiO.sub.2, 9-12% Al.sub.2 O.sub.3, and 9-12% SrO are disclosed. These glasses exhibit the high annealing points needed for processing into display devices, and also exhibit high viscosity at the liquidus temperature so that they can be formed by overflow downdraw sheet forming methods such as described in U.S. Pat. No. 3,338,696. However, they must be melted and formed at relatively high temperatures, which are beyond the proven capability of presently existing large-scale melting equipment.
As pointed out in the latter two patents, present liquid crystal display (LCD) technology involves the application of large arrays of thin film transistors directly to the surface of the glass. Such transistor arrays have been found to be essential in order to provide displays exhibiting rapid switching response to electrical signals.
To provide the required thin film transistor (TFT) array, the transistors are grown in situ on a sheet of the substrate glass which has been pretreated to provide a polysilicon layer thereon. However the development of such a layer demands a glass with a rather high strain point temperature because of the elevated processing temperatures which are customarily used. In fact, as the display technology has progressed, glasses exhibiting higher and higher strain points have been required. At the same time, however, the thermal expansion characteristics needed to insure physical compatibility between the glass and the polysilicon support layer have had to be maintained, as has the excellent chemical durability required for TFT array development and support.
The various requirements for glasses to be used for LCD display applications employing thin film transistor array technology, and which must be combined in a single glass, may be summarized as follows:
(1) the glass must be substantially free of intentionally added alkali in order to avoid the possibility that alkali from the glass substrate could migrate into the transistor matrix;
(2) the glass substrate must be sufficiently chemically durable to withstand the reagents used in the TFT matrix deposition process;
(3) the expansion mismatch between the glass substrate and the silicon present in the TFT array must be maintained or even reduced as processing temperatures for these substrates increase; and
(4) the glass must be producible in high quality sheet form at low cost, and thus should not require extensive grinding and polishing to achieve the necessary surface finish.
This last requirement implies that a sheet glass manufacturing process capable of producing essentially finished glass sheet, such as an overflow downdraw sheet manufacturing process, must be used. This in turn demands a glass with a high viscosity at the liquidus; the minimum liquidus viscosity for stable long-term overflow downdraw sheet forming is presently considered to be about 3.times.10.sup.5 poises.
A commercially available glass, presently used for the fabrication of liquid crystal display devices, is Corning Code 7059 glass. This glass, consisting of about 50% SiO.sub.2, 15% B.sub.2 O.sub.3, 10% Al.sub.2 O.sub.3, and about 24% BaO by weight, is nominally alkali-free, has an expansion of about 46.times.10.sup.-7 .degree. C., and exhibits a viscosity somewhat in excess of 10.sup.6 poises at the liquidus temperature. The high liquidus viscosity of this glass facilitates the production of glass sheet therefrom by overflow downdraw sheet processing. However, the relatively low strain point (approximately 590.degree. C.) of this glass only marginally meets the demands of advanced LCD display processing technology.
To withstand present display processing a glass strain point of at least 625.degree. C. is considered to be required, and the chemical durability of the glass should also be substantially improved. However, it is difficult to increase the strain point of alkali-free glasses of known types without undesirably raising the liquidus temperature of the glass beyond a level which is essential for the efficient and economical manufacture of glass sheet. Nevertheless, useful improvements in glass processability for such displays would require glasses exhibiting not only the higher strain point, but also an average linear coefficient of thermal expansion in the range of about 20-60.times.10.sup.-7 /.degree. C., a viscosity at the glass liquidus temperature in excess of about 3.times.10.sup.5 poises, and a chemical durability characterized by a weight loss in 5% (weight) aqueous HC1 at 95.degree. C. not exceeding about 10 mg/cm.sup.2 of glass surface area over an interval of 24 hours.